"Tumor Cell Hypoxia as a Factor in Cancer Therapy" is a long range project aimed at promoting a more quantitative analysis of factors that affect the radiation response of mammalian cells. The central theme for the proposed studies is to investigate the basic mechanisms of action for hypothermia, electron-affinic radiosensitizers, and radiation, to define the interactions between these three treatment modalities at the cellular and molecular level, and to utilize this information for optimizing treatment strategies for cancers. Using a variety of in vivo and in vitro procedures (colony assay, serial dilution assay, 125 I-iododeoxyuridine prelabeling technique), an attempt will be made to evaluate the importance of the oxygen effect in radiation therapy of cancers. Specifically, the effects of three classes of electron-affinic agents (nitroimidazoles, benzotriazines, quinones) will be investigated on euoxic and hypoxic cancer cells at normal (37 degrees Celsius) and elevated (41.5 degrees Celsius) incubation temperatures. We have previously demonstrated that exposure of tumors to physiologically tolerable doses of nitroimidazoles (misonidazole, metronidazole) alone or hyperthermia alone produces only modest radiosensitization (enhancement ratio up to 2.0). However, combined administration of hyperthermal and nitroimidazoles induces very pronounced, synergistic radiosensitization effects with enhancement ratios of 4.3 or higher. This ratio is considerably better than the 3-fold sensitization that could be achieved by full oxygenation of previously hypoxic tumors. Since identical combination treatment produces only modes or no radiosensitization of normal body tissues, the proposed treatment regimen could produce a large therapeutic gain factor in cancer radiation therapy. The immediate goal is to extend this work to newly developed electron- affinic agents, to further investigate the cellular and molecular mechanisms responsible for synergistic radiosensitization (dose modification, repair inhibition, additive damage interactions), to evaluate the mechanisms responsible for direct cytocidal effects of hyperthermia and electron-affinic agents, and to obtain additional information on environmental and intracellular factors which govern the magnitude of these effects. As part of this project, ongoing radionuclide suicide studies with iodine-125 and other Auger emitting radionuclides will be continued. The aim of these studies is to explore the fundamental mechanism(s) for radiation-induced cell death and to utilize the unique decay properties of Auger emitters for selective radionuclide therapy of cancers.